A great plot could involve a tournament in the High School DxD world that Gohan participates in. He teams up with some of the characters from that series. His different fighting style and abilities would shock everyone. There could be some sort of mystery or conspiracy behind the tournament as well, and Gohan's integrity and sense of justice would drive him to uncover the truth.
You could do a crossover with 'Sword Art Online'. The virtual reality aspect of SAO could be combined with the supernatural elements of High School DxD. For example, the characters could be trapped in a virtual world that has elements of both worlds, and they have to fight their way out. Issei might have to use his skills in a new digital environment while also dealing with SAO - like challenges.
In a Gohan in High School DxD fanfiction, Gohan could be a special exchange student. He has to balance his studies and his new life in this strange world filled with devils and angels. He might start a martial arts club in the school, which becomes popular. But his real purpose could be to find a way to bring peace between the different factions, using his Saiyan abilities and kind heart.
One idea could be the characters from Kid Icarus and High School DxD teaming up to fight a common supernatural threat that combines the mythologies of both universes. For example, a demon - angel hybrid that also has ties to the Greek mythology in Kid Icarus.
Well, a cool crossover idea might be the sharing of powers. Imagine if some of the High School DxD characters could learn Naruto's shadow clone jutsu. It would be really useful in battles. Also, the idea of Naruto characters trying to understand the complex social hierarchy in DxD could be very entertaining. They could end up forming alliances in unexpected ways, like the ninjas teaming up with the devils to fight against a powerful demon from Naruto's world that has somehow entered the DxD realm.
In Gohan High School DxD fanfiction, there could be a concept of Gohan's academic influence. Gohan is a very intelligent character. He might introduce new scientific ideas or study techniques to the students at the high school in the DxD world. This could lead to different character developments, as some characters might be inspired to pursue knowledge more seriously, and it could also affect the relationships between different factions in the story, as knowledge could be seen as a form of power.
One idea could be Gohan arriving in the DxD world during a major crisis. His Saiyan powers would shock the devils and angels. Maybe he teams up with Issei to face a powerful common enemy.
One idea could be a crossover with 'Fate/stay night'. You could have the characters from High School DxD enter the Holy Grail War. Issei could use his boosted gear in new and exciting ways against the Servants. Another idea is a crossover with 'Naruto'. The ninjutsu from Naruto's world would be a cool addition to the battles in High School DxD. For example, Sasuke's Sharingan could create some interesting interactions with the devil eyes in DxD.
A plot could involve Kaido being sent on a special mission by the Devil King to infiltrate a human school. During this, he starts to develop real friendships with the characters in High School DxD. His initial motives are questioned as he begins to care more about his new friends than his mission, creating a lot of internal conflict.
One idea could be a Dragonborn character arriving in the High School DxD world and having to adapt to the unique school and supernatural environment. They might use their dragonborn abilities to stand out in the Rating Games.
One plot idea could be that Naruto is summoned to the High School DxD world by a powerful magic user who mistakes him for a powerful demon. Once there, Naruto has to prove that he is not a threat while also trying to find a way back home. Along the way, he befriends some of the students at the school and helps them with their own problems, like protecting the school from a sudden attack by a group of rogue devils.